The present invention relates to interfacing color component signals having variable black and white levels, for driving a display device after linear amplification, with a digital signal processor. The present invention may be employed in a display apparatus, for example where the display device is a cathode ray tube (CRT).
In known types of display apparatus, a received composite video signal, is processed to derive color component signals. The initially derived color component signals have fixed black and white levels, typically 0.7V apart, and are subsequently amplified to derive color component signals each having black and white levels which vary unpredictably and independently. For example, FIG. 1 shows a line of period H of a color component signal comprising a horizontal blanking interval 1 below a black level Vblack and a signal 2 representing a line of the image. The signal 2 varies between the black level Vblack and a white level Vwhite depending on the picture content. The black and white levels may be considered as notional levels because the color component signal might or might not reach them on any given line
The black level might vary between two limits, typically 0.3V and 3.0V, depending on: the brightness set by the user; the characteristics of the CRT; and a gain control feedback signal that varies the amplifier gain over the lifetime of the CRT to maintain picture quality. The white level might vary between two limits, typically 0.3V and 3.3V above the black level, depending on: the initial drive levels set in the display apparatus for the respective color component signals; the contrast, chroma and hue set by the user; the characteristics of the CRT; and the feedback gain control. Thus the black and white levels of each color component signal output by the video signal processing circuit vary unpredictably and independently from each other.
It is desirable to interface the color component signals which have variable black and white levels with a digital signal processor to perform various types of processing to modify the characteristics of the image. However, in so doing, it is critical to retain the levels of the color component signals supplied to a display device via a linear amplifier to prevent distortion of the color balance in the displayed picture.
Theoretically, the color component signals could be supplied directly to a digital signal processor through an analog-to-digital (A/D) convertor. However, to maintain the necessary accuracy in the levels of the color component signals, the resolution of the system would need to be very high, say around 1 mV. Given that the white level can vary over a range of 6.3V, the accuracy of the A/D convertor would need to be (1xc3x9710xe2x88x923)/6.3 or 0.016%. To achieve this accuracy, a 13 bit A/D convertor would be required. However, a 13 bit A/D convertor operating at video frequencies is very difficult with modern integrated circuit technology and cannot currently be achieved using cheap mass production technology. The only suitable 13 bit A/D convertors currently available are unacceptably expensive.
According to one aspect of the present invention, there is provided an interface circuit for interfacing color component signals, which have variable black and white levels and which color component signals are for driving a display device after linear amplification, with a digital signal processor, the interface circuit comprising: a modification circuit arranged to receive color component signals which have variable black and white levels, to perform a modification of the voltage levels of the received color component signals, and to output the modified color component signals for supply to a digital signal processor via an A/D convertor; and a remodification circuit arranged to receive processed color component signals supplied from the digital signal processor via a D/A convertor, to perform a modification of the processed color component signals which is the inverse of the modification performed by the modification circuit, thereby restoring the voltage levels of the color component signals, and to output the restored color component signals for supply to a display device via a linear amplifier.
Accordingly, the interface circuit is capable of providing a highly accurate representation of the color component signals whilst reducing the resolution necessary in the A/D conversion, because the modification performed by the remodification circuit is the inverse of the modification performed by the modification circuit. For example, in the described embodiment, the A/D convertor may be 8 bit and hence easily implemented, whilst still providing a resolution for the picture content which is adequate for the display device.
Preferably, calibration signals including at least one level fixed relative to the variable black and white levels are inserted in a predetermined period in each color component signal. Then the modification and remodification circuits may be arranged to perform the modifications using the calibration signals. The use of such calibration signals allows a high degree of accuracy to be maintained in the modifications performed in the interface circuit.
The predetermined period in which the calibration signals are inserted may be in the vertical blanking interval, so that the picture displayed on the display device is not disrupted.
Desirably, the calibration signal includes the black level of the respective color component signal, the interface circuit includes a detector for detecting the black levels of the respective color component signals from the calibration signals, the modification circuit is arranged to subtract the detected black levels from the respective, received color component signals, and the remodification circuit is arranged to add the same, detected black levels to the respective, processed color component signals.
By supplying the black levels in the calibration signals, the interface circuit can perform the modifications based on the actual black level, despite variation in that black level over time. Subtraction of the detected black level from the respective color component signals causes each color component signal to be shifted or re-referenced to a constant known level regardless of the actual black level. As the remodification circuit adds the same, detected black level which is subtracted in the modification circuit, any degree of inaccuracy in the detected black level does not affect the restored color component signals.
Preferably, the detector comprises, in respect of each of the color component signals, an analog sample-and-hold circuit controlled to sample the black level of the respective color component signal from the respective calibration signal in said predetermined period and thereafter to hold the black level. This has the advantages of smaller size and higher accuracy over the alternative of an A/D convertor.
Preferably, the sample-and-hold circuits are arranged to sample the calibration signals every predetermined period of the color component signals and the modification and remodification circuits operate in vertical synchronism. In this manner, if there is droop over a vertical period in the detected black level held by the sample-and-hold circuit, there is no effect on the restored color component signals, because the same droop is present in the black level subtracted and added at any given point in the picture.
Preferably, the modification circuit includes a blanking removal circuit arranged to replace the horizontal blanking intervals of the color component signals by the respective detected black levels, and the remodification circuit includes a blanking insertion circuit for re-inserting horizontal blanking intervals into the processed color component signals. Accordingly, the blanking interval of the color component signals, when the signal level is below the black level, is not supplied to the A/D convertor or digital signal processor.
Desirably, the calibration signal includes a scaling level above the black level, preferably at the white level. Then the modification circuit may include, in respect of each color component signals, a scaling circuit arranged to scale the respective color component signal by a respective scaling factor selected using a scaling level in the respective calibration signal to normalise the range between the white and black levels, and the remodification circuit may include, in respect of each color component signal, a re-scaling circuit arranged to re-scale the color component signals by a respective re-scaling factor which is the inverse of the scaling factor used in the respective scaling circuit.
As a result, regardless of the actual white level, the color component signals are scaled to normalise the range between the white and black levels. As the possible range of the color component signals is normalised to a known range, the necessary dynamic range of the A/D convertor is reduced to that known range. As a result, the number of bits of the A/D convertor may be reduced. For example, the range between the white and black levels which is variable between 0.3V and 3.3V may be normalised to 1.0V. The remodification circuit restores the original range by re-scaling the color component signals by a re-scaling factor which is the inverse of the scaling factor.
Preferably, the interface circuit further includes a bypass circuit for passing the received color component signals to the output of the interface circuit, bypassing the modification and remodification circuits, during predetermined horizontal periods of the color component signals when the color component signals consist of color test signals of fixed amplitude relative to the black and white levels.
Color test signals of fixed amplitude relative to the black and white levels may be inserted on predetermined horizontal periods for use by a feedback gain control circuit. The outputs of the linear amplifier derived from the test signals are monitored and fed back as a control signal to the video signal processing circuit to control the gain of the amplifiers which produce the color component signals to compensate for decay in the properties of the display device over its lifetime. The bypass circuit allows the test signals to bypass the modification and remodification circuits, and hence also the digital signal processor. This prevents the test signals from being disrupted in any way. This is highly advantageous, because a high degree of accuracy is needed in the test signals to allow the feedback control to work effectively. Otherwise, a high resolution A/D convertor would have been necessary to maintain the accuracy of the test signals during passage through the interface circuit and the digital signal processor.
According to further aspects of the invention, there are provided corresponding methods of inserting calibration signals in color component signals and interfacing the color component signals with a digital signal processor.